We compare magnetic field extrapolations from a photospheric
magnetogram with the observationally inferred structure of magnetic loops in a
newly developed active region. This is the first time that the reconstructed
3D-topology of the magnetic field is available to test the extrapolations. We
compare the observations with potential fields, linear force-free fields and
non-linear force-free fields. This comparison reveals that a potential field
extrapolation is not suitable for a reconstruction of the magnetic field in
this young, developing active region. The inclusion of field-line-parallel
electric currents, the so called force-free approach, gives much better
results. Furthermore, a non-linear force-free computation reproduces the
observations better than the linear force-free approximation, although no free
parameters are available in the former case.

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